1. Field Of The Invention
The invention relates to hydraulic reaction turbines and centrifugal pumps; and, more particularly, to apparatus and method for reducing the effects of draft tube pressure fluctuations acting on a runner in a hydraulic reaction turbine or the effects of suction tube pre-rotation acting on the impeller in a centrifugal pump.
2. Description Of The Prior Art
Hydraulic reaction turbines have been known in the art for many years. In the early development of such turbines, it was discovered that turbines which were operated at heads and flows other than the design point of the turbine, experienced rough operation and power swings resulting from water passing out through the draft tube of the turbine in a whirling and spiralling vortex. This is particularly true of Francis-type turbines, propeller turbines and pump turbines. In Apr. 1940, W. J. Rheingans published a paper in the Transactions of the American Society of Mechanical Engineers in which he outlined the results of an extensive study of the draft tube surging phenomenon in turbines. In his paper, and in the comments from others, several schemes for suppressing draft tube surging were discussed; these included fins projecting from the draft tube walls, an extended runner cone filling the space occupied by the draft tube vortex, venting of the turbine head cover and air injection into the draft tube through the straightening fins and through a hole in the draft tube wall. Since that time there have been numerous papers on the subject and many suggested schemes to reduce draft tube surging. Such schemes have included air admission to the turbine runner, to the draft tube, to the lower side of the runner through the runner cone, or through holes in streamlined shapes placed across the draft tube. Air has also been injected into the penstock, into the space between the wicket gates and the runner, and through the runner band. There have also been many schemes for the use of fins in the draft tube; these include flow splitters and a coaxial hollow cylinder placed below the center of the runner cone and supported by fins which can be rotated, and several types of fins as discussed hereinabove.
When a hydraulic reaction turbine is operated at a head and/or flow which is different from the design point, the water will follow a helical path as it passes out through the draft tube. If the operating conditions are sufficiently remote from the design point, a vortex will form in the swirling water just below the runner cone and this vortex usually will follow a helical path as it passes out through the draft tube. The core of the vortex is usually filled with water vapor. The spiralling vortex causes pressure fluctuations which vary the net head experienced by the runner and cause the generated power to fluctuate in a similar manner. Net head is also affected when the water vapor bubbles in the core of the vortex collapse.
A similar problem exists in centrifugal pumps. Centrifugal pumps are designed to operate with a given quantity of flow and against a given head. When a centrifugal pump is operated at its design point, the water flowing through the machine is laminar, irrotational flow, and the pump runs smoothly. Centrifugal pumps have been in use for many years. Early in their development, it was known that prerotation occurred in the suction tube if a pump was operated at a head and/or quantity of flow different from the design values. This phenomenon has been noticed especially in large pumps with Francis-type impellers. It has also been suspected in some large Deriaz-type machines. If the operating conditions are sufficiently remote from the design point, a vortex will form in the center of the water flowing in through the suction tube and usually will follow a helical path. This vortex flow causes a rotating low pressure area under the pump impeller which can induce destructive vibration of the pump, pump shaft, motor shaft and the motor.
There is thus a need for reducing the effects of draft tube pressure fluctuations acting on a runner in a hydraulic reaction turbine or the effects of suction tube pre-rotation acting on the impeller in a centrifugal pump.